Semiconductor device and method of forming the same

ABSTRACT

A semiconductor device includes a storage element layer and a selector. The selector is electrically coupled to the storage element layer, and includes a first insulating layer, a second insulating layer, a third insulating layer, a first conductive layer and a second conductive layer. The first insulating layer, the second insulating layer and the third insulating layer are stacked up in sequence, wherein the second insulating layer is sandwiched in between the first insulating layer and the third insulating layer, and the first insulating layer and the third insulating layer include materials with higher band gap as compared with a material of the second insulating layer. The first conductive layer is connected to the first insulting layer, and the second conductive layer is connected to the third insulating layer.

BACKGROUND

A magnetic random access memory (MRAM) offers comparable performance tovolatile static random access memory (SRAM) and comparable density withlower power consumption to volatile dynamic random access memory (DRAM).Compared to non-volatile memory flash memory, an MRAM offers much fasteraccess times and suffers minimal degradation over time, whereas a flashmemory can only be rewritten a limited number of times. In general, MRAMstores data using storage elements such as magnetic tunnel junction(MTJ), and the MTJ is typically coupled in series with a control devicesuch as a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the criticaldimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1A to FIG. 1E are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with someembodiments of the present disclosure.

FIG. 2A to FIG. 2D are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with some otherembodiments of the present disclosure.

FIG. 3A to FIG. 3F are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with some otherembodiments of the present disclosure.

FIG. 4A to FIG. 4E are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with some otherembodiments of the present disclosure.

FIG. 5 is a schematic sectional view of a semiconductor device inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a second feature over or on a first feature in the description thatfollows may include embodiments in which the second and first featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the second and first features,such that the second and first features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath”, “below”, “lower”,“on”, “over”, “overlying”, “above”, “upper” and the like, may be usedherein for ease of description to describe one element or feature'srelationship to another element(s) or feature(s) as illustrated in thefigures. The spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Magnetic random access memory (MRAM) stores data using storage elementssuch as a magnetic tunnel junction (MTJ). A typical MTJ device used inan MRAM cell includes an MTJ coupled in series with a control devicesuch as a transistor. However, for transistor coupled in series, scalingis compromised given the high current needed to operate an MRAM array.Furthermore, in order to reliably access the memory, the transistorparameters are tuned in such a way so as to allow sufficient current toflow when the MTJ needs to be selected (transistor on), and not allowcurrent flow when the cell is not selected or half selected (transistoroff). However, leakage current (sneak paths) is generally inevitablyobserved in the non-selected and half selected cells. Therefore, aselector with smaller footprint is needed to replace the accesstransistor for better area scaling, and is needed for minimizing theleakage current. In the embodiments of present disclosure, asemiconductor device comprising a storage element layer (the MTJ)electrically coupled to a selector is described. The selector enablesfull control of leakage current, threshold voltage and maximum currentdensity independently, and is optimized for MRAM devices.

FIG. 1A to FIG. 1E are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with someembodiments of the present disclosure. Referring to FIG. 1A, a firstinterconnection structure 102 is provided. The first interconnectionstructure 102 may be formed over a substrate (not shown) of asemiconductor device for providing interconnection between elements. Thefirst interconnection structure 102 may include an insulating layer 102Aand a conductive via 102B disposed therein. The insulating layer 102Amay be formed by any suitable method, such as chemical vapor deposition(CVD), or the like. The conductive via 102B may be formed by patterningthe insulting layer 102A to form openings; filling the openings withconductive materials; and performing a planarization step (e.g.chemical-mechanical planarization; CMP) to form the conductive via 102Bhaving a top surface that is levelled with a top surface of theinsulating layer 102A.

In some embodiments, the insulating layer 102A is referred to as aninter-metal dielectric (IMD) layer which may be made of a dielectricmaterial, such as silicon oxide, silicon nitride, silicon oxynitride, aspin-on dielectric material, or a low-k dielectric material. It shouldbe noted that the low-k dielectric materials are generally dielectricmaterials having a dielectric constant lower than 3.9. In certainembodiments, the conductive via 102B include commonly used conductivematerials, such as metals or metal alloys including one or more of Al,AlCu, Cu, Ti, TiN, W, and the like. The first interconnection structure102 including the insulating layer 102A and the conductive via 102Bforms a portion of a current driving circuit (not shown) to providecurrent to the storage element layer described subsequently.

Referring to FIG. 1B, after forming the first interconnection structure102, an insulating layer 104A and a bottom electrode 104B embedded inthe insulating layer 104A are formed over the first interconnectionstructure 102. For example, the bottom electrode 104B is formed to be incontact with the conductive via 102B of the first interconnectionstructure 102. A material of the insulating layer 104A may be similar tothe insulating layer 102A shown in FIG. 1A. For example, the insulatinglayer 104A is made of a dielectric material, such as silicon oxide,silicon nitride, silicon oxynitride, a spin-on dielectric material, alow-k dielectric material, or the like. In some embodiments, the bottomelectrode 104B is made of conductive materials, such as Ti, Ta, TaN, Co,Cu, AlCu, W, TiN, TiW, TiAl, TiAlN, Pt, Ru, RuO_(x), Mo, MoO_(x), or acombination thereof. The bottom electrode 104B may be formed by anysuitable method, such as chemical vapor deposition (CVD), physical vapordeposition (PVD), or the like.

After forming the insulting layer 104A and the bottom electrode 104B, astorage element layer 106, a top electrode 108, a selector 200 and ahard mask layer 208 are sequentially disposed on the bottom electrode104B. In some embodiments, the storage element layer 106 is formed onthe bottom electrode 104B to be in contact with the bottom electrode104B. In certain embodiments, the storage element layer 106 is referredto as a magnetic tunnel junction (MTJ). The formation of the storageelement layer 106 (or the MTJ) includes sequentially forming a referencelayer 106A, a tunnel barrier layer 106B and a free layer 106C over thebottom electrode 104B.

In some embodiments, the reference layer 106A and the free layer 106Cinclude one or more ferromagnetic materials that can be magneticallyoriented, respectively. For example, in one embodiment, a magneticorientation of the reference layer 106A is static (i.e., fixed), while amagnetic orientation of the free layer 106C is capable of switchingbetween a parallel configuration and an anti-parallel configuration withrespect to that of the reference layer 106A. The parallel configurationprovides for a low resistance state that digitally stores data as afirst data state (e.g., a logical “0”). The anti-parallel configurationprovides for a high resistance state that digitally stores data as asecond data state (e.g., a logical “1”). In some embodiments, the tunnelbarrier layer 106B is made of a non-magnetic material and is disposedbetween the reference layer 106A and the free layer 106C. In certainembodiments, the tunnel barrier layer 106B includes a relatively thinoxide layer capable of electrically isolating the free layer 106C fromthe reference layer 106A at low potentials and capable of conductingcurrent through electron tunneling at higher potentials. In oneembodiment, the tunnel barrier layer 106B is made of magnesium oxide(MgO).

As illustrated in FIG. 1B, the top electrode 108 is formed on thestorage element layer 106. For example, the top electrode 108 is formedon and in contact with the free layer 106C of the storage element layer106, while the storage element layer 106 is disposed in between the topelectrode 108 and the bottom electrode 104B. In some embodiments, thetop electrode 108 includes a conductive material, such as Ti, Ta, TaN,Co, Cu, AlCu, W, TiN, TiW, TiAl, TiAlN, Pt, Ru, RuO_(x), Mo, MoO_(x), ora combination thereof. The top electrode 108 and the bottom electrode104B may include the same material or different materials. For example,in one embodiment, the top electrode 108 and the bottom electrode 104Bare both made of TiN. The top electrode 108 may be formed by anysuitable method, such as chemical vapor deposition (CVD), physical vapordeposition (PVD), or the like.

After forming the top electrode 108, the selector 200 is formed over thetop electrode 108 to be connected to the top electrode 108. In someembodiments, the selector 200 is electrically connected to the storageelement layer 106 through the top electrode 108. In some embodiments,the selector 200 is formed by sequentially forming a first conductivelayer 202, insulating layers 204 and a second conductive layer 206 overthe top electrode 108. The first conductive layer 202 and the secondconductive layer 206 are made of metals having a work function of 3.8 eVto 4.6 eV. For example, the first conductive layer 202 and the secondconductive layer 206 may include materials such as Al, Ti, TiN, TiW,TiAl, Ta, Hf, W, WN, Mo, Zr, Mg, V, Nb or the like. The first conductivelayer 202 and the second conductive layer 206 may be formed by anysuitable method, such as chemical vapor deposition (CVD), atomic layerdeposition (ALD), physical vapor deposition (PVD), or the like.Furthermore, the first conductive layer 202 and the second conductivelayer 206 may include the same materials or include different materials.

As illustrated in FIG. 1B, the insulating layers 204 are formed betweenthe first conductive layer 202 and the second conductive layer 206. Theformation of the insulating layers 204 includes sequentially forming afirst insulating layer 204A, a second insulating layer 204B and a thirdinsulating layer 204C stacked over the first conductive layer 202. Theinsulating layers 204 may be formed by any suitable method, such aschemical vapor deposition (CVD), physical vapor deposition (PVD), or thelike. In some embodiments, the first insulating layer 204A is connectedto and in contact with the first conductive layer 202. In someembodiments, the second insulating layer 204B is sandwiched in betweenthe first insulating layer 204A and the third insulating layer 204C. Incertain embodiments, the third insulating layer 204C is connected to andin contact with the second conductive layer 206.

In the exemplary embodiment, the first insulating layer 204A and thethird insulating layer 204C include materials with higher band gap ascompared with a material of the second insulating layer 204B. In otherwords, a low band gap material layer (the second insulating layer 204B)is disposed in between two high band gap material layers (the firstinsulting layer 204A and the third insulating layer 204C). In someembodiments, the first insulting layer 204A and the third insulatinglayer 204C (the high band gap materials) include materials selected fromthe group consisting of silicon nitride (SiN), silicon oxide (SiO₂),aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃) and hafnium silicate(HfSiO_(x)). Furthermore, the second insulating layer 204B (the low bandgap materials) includes a material selected from the group consisting oftitanium oxide (TiO₂), strontium titanate (SrTiO₃), barium titanate(BaTiO₃), zirconium oxide (ZrO₂), hafnium oxide (HfO₂) and hafniumtitanate (HfTiO_(x)). In one embodiment, the first insulting layer 204Aand the third insulating layer 204C include the same high band gapmaterials, but the disclosure is not limited thereto. In an alternativeembodiment, the first insulting layer 204A and the third insulatinglayer 204C include different high band gap materials.

Furthermore, in some embodiments, a thickness T1 of the first insultinglayer 204A and a thickness T3 of the third insulating layer 204C issmaller than or equal to a thickness T2 of the second insulating layer204B. The thickness T1 being substantially equal to the thickness T3. Inone embodiment, the thickness (T1, T3) of each of the first insultinglayer 204A and the third insulating layer 204C relative to the thicknessT2 of the second insulating layer 204B is 1:1 to 1:50. In certainembodiments, the thickness (T1 and T3) of each of the first insultinglayer 204A and the third insulating layer 204C relative to the thicknessT2 of the second insulating layer 204B is 1:2 to 1:40. In someembodiments, the thickness (T1 and T3) of each of the first insultinglayer 204A and the third insulating layer 204C is in a range of 0.1 nmto 1 nm. In some embodiments, the thickness T2 of the second insulatinglayer 204B is in a range of 1 nm to 5 nm. In some embodiments, thethickness (T1 and T3) of each of the first insulting layer 204A and thethird insulating layer 204C is controlled in the above range so that thetunneling current is high enough to be able to switch the storageelement layer 106 (the MTJ). In some embodiments, the thickness T2 ofthe second insulating layer 204B defines at which voltage the currentwill start to increase rapidly to enhance the performance of theselector 200.

In some embodiments, the first conductive layer 202, the insulatinglayers 204 (three insulating layers 204A-204C) and the second conductivelayer 206 of the selector 200 are respectively Al, Al₂O₃, SrTiO₃, Al₂O₃and Al. In some alternative embodiments, the first conductive layer 202,the insulating layers 204 (three insulating layers 204A-204C) and thesecond conductive layer 206 of the selector 200 are respectively TiN,Al₂O₃, SrTiO₃, Al₂O₃ and TiN.

In the exemplary embodiment, by using a selector 200 including the firstconductive layer 202 and the second conductive layer 206 having theselected work function, and including the insulating layers 204 havingthe low and high band gap materials, better control of the on/offcurrent of the selector 200 may be achieved. For example, in someembodiments, when an applied voltage is smaller than a differencebetween a work function of the conductive layers (202, 206) and anelectron affinity of the second insulating layer 204B (low appliedvoltage), the current flow is low because electrons sees the energybarrier of all the three insulating layers (204A-204C) (controls the“off” current). In some embodiments, when the applied voltage is greaterthan a difference between a work function of the conductive layers (202,206) and an electron affinity of the second insulating layer 204B, butsmaller than a difference between a work function of the conductivelayers (202, 206) and an electron affinity of the first or thirdinsulating layer 204A or 204C (high applied voltage), then the electronssee the barriers of the outer insulating layers (204A, 204C), and thecurrent flow increases rapidly and the electrons tunnels through thethree insulating layers (204A-204C) (controls the “on” current;corresponding to the threshold voltage). Furthermore, since the appliedvoltage is greater than a difference between a work function of theconductive layers (202, 206) and an electron affinity of the secondinsulating layer 204B, this further increases the current flow and theelectrons tunnels through a triangular barrier by Fowler-Nordheimtunneling through the second insulating layer 204B. In some embodiments,when the applied voltage is higher than the difference between a workfunction of the conductive layers (202, 206) and an electron affinity ofthe first or third insulating layer 204A or 204C, then the current flowis high. As such, a selector 200 having relatively high resistance tocurrent flow at low bias and very high current flow at high bias can beachieved. Furthermore, the selector 200 enables full control of theleakage current as the current passing through a selected cell exceedsthe residual leakage.

Referring to FIG. 1B, after forming the selector 200, a hard mask layer208 is formed over the selector 200. For example, the hard mask layer208 is formed on and in contact with the second conductive layer 206 ofthe selector 200. In some embodiments, the hard mask layer 208 is ametal hard mask made of materials such as TiN, TaN, or the like.However, the disclosure is not limited thereto. In some alternativeembodiments, the hard mask layer 208 is an inorganic dielectricmaterial, such as SiON, SiN, SiC, SiOC, SiCN, or a combination thereof.The hard mask layer 208 may be formed by any suitable method, such aschemical vapor deposition (CVD), physical vapor deposition (PVD), or thelike. At this stage, the hard mask layer 208 has sidewalls that arealigned with the selector 200, the top electrode 108, the storageelement layer 106 and the insulating layer 104A.

Referring to FIG. 1C, in a next step, the storage element layer 106, thetop electrode 108, the selector 200 and the hard mask layer 208 areselectively etched to form a stacked structure having inclinedsidewalls. For example, the selective etching process includes areactive ion etching (RIE) process, an ion beam etching (IBE) process,or combinations thereof. In some embodiments, after the selectiveetching process, the inclined sidewalls 200SD of the selector 200 arealigned with the inclined sidewalls 106SD of the storage element layer106.

Referring to FIG. 1D, after performing the selective etching process, aliner layer 110 is conformally formed over the insulating layer 104A andover the hard mask layer 208. For example, the liner layer 110 coversthe inclined sidewalls 200SD of the selector 200 and covers the inclinedsidewalls 106SD of the storage element layer 106. In some embodiments,the liner layer 110 includes a dielectric material such as siliconoxide, silicon nitride, and/or other suitable dielectric materials.Furthermore, the liner layer 108 may be formed by a suitable processsuch as atomic layer deposition (ALD), chemical vapor deposition (CVD),physical vapor deposition (PVD), or the like.

As illustrated in FIG. 1D, after forming the liner layer 110, apassivation layer 112 surrounding the liner layer 110 is formed. In someembodiments, the passivation layer 112 is formed on the liner layer 110and over the insulating layer 104A. In certain embodiments, thepassivation layer 112 surrounds the storage element layer 106, the topelectrode 108, the selector 200 and the hard mask layer 208. In someembodiments, the passivation layer 112 is referred to as an inter-metaldielectric (IMD) layer which includes a dielectric material, such assilicon oxide, silicon nitride, silicon oxynitride, a spin-on dielectricmaterial, or a low-k dielectric material. It should be noted that thelow-k dielectric materials are generally dielectric materials having adielectric constant lower than 3.9. Furthermore, the passivation layer112 may be formed by any suitable method, such as chemical vapordeposition (CVD), or the like.

Referring to FIG. 1E, in a next step, portions of the passivation layer112 and portions of the liner layer 112 are removed to form an opening(not shown) revealing a surface of the hard mask layer 208 (metal hardmask). For example, the passivation layer 112 and the liner layer 112may be removed by etching processes, or other suitable processes.Thereafter, a second interconnection structure 114 is formed in theopening over the hard mask layer 208, and the second interconnectionstructure 114 is electrically connected to the selector 200 through thehard mask layer 208. In some embodiments, the second interconnectionstructure 114 is formed by filling a conductive material in the openingof the passivation layer 112, then performing a planarization process(e.g. a chemical-mechanical planarization (CMP) process) to removeexcess conductive material. In some embodiments, the secondinterconnection structure 114 include metals or metal alloys such as oneor more of Al, AlCu, Cu, Ti, TiN, W, or the like. After forming thesecond interconnection structure 114, a semiconductor device SM1according to some embodiments of the present disclosure is accomplished.

As illustrated in FIG. 1E, the semiconductor device SM1 includes astorage element layer 106 and a selector 200 electrically coupled to thestorage element layer 106. For example, the selector 200 is electricallyconnected to the storage element layer 106 through the top electrode108. In some embodiments, the first interconnection structure 102 iselectrically connected to the storage element layer 106 (the MTJ)through the bottom electrode 104B. In certain embodiments, the secondinterconnection structure 114 is electrically connected to the selector200 through the hard mask layer 208 (metal hard mask). In somealternative embodiments, the hard mask layer 208 may be optionallyremoved, and the second interconnection structure 114 is electricallyconnected and in contact with the second conductive layer 206 of theselector 200. In the semiconductor device SM1, by using the selector 200electrically coupled (in series) to the storage element layer 106 (theMTJ), a device that enables full control of leakage current, thresholdvoltage and maximum current density independently can be achieved.

FIG. 2A to FIG. 2D are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with some otherembodiments of the present disclosure. The method illustrated in FIG. 2Ato FIG. 2D is similar to the method illustrated in FIG. 1A to FIG. 1E.Therefore, the same reference numerals are used to refer to the same orliked parts, and its detailed description will be omitted herein.

In the method of FIG. 1A to FIG. 1E, the storage element layer 106 isformed prior to forming the selector 200, and the selector 200 is formedover the top electrode 108 so that the first conductive layer 202 of theselector 200 is connected to the top electrode 108. However, thedisclosure is not limited thereto, and in the method of FIG. 2A to FIG.2D, the storage element layer 106 is formed after forming the selector200.

Referring to FIG. 2A, in some embodiments, the selector 200 is formedover the first interconnection structure 102 and connected to the firstinterconnection structure 102. For example, the selector 200 is formedby sequentially forming a first conductive layer 202, insulating layers204 (a first insulating layer 204A, a second insulating layer 204B and athird insulating layer 204C) and a second conductive layer 206 stackedup in sequence. The first conductive layer 202 of the selector iselectrically and physically connected to the conductive via 102B of thefirst interconnection structure 102.

After forming the selector 200, a bottom electrode 104B is formed overthe selector 200, wherein the bottom electrode 104B is electrically andphysically connected to the second conductive layer 206 of the selector200. Subsequently, a storage element layer 106 (the MTJ) inclusive of areference layer 106A, a tunnel barrier layer 106B and a free layer 106Cis formed over the bottom electrode 104B. For example, the referencelayer 106A is disposed on and connected to the bottom electrode 104B.Thereafter, a top electrode 108 and a hard mask layer 208 aresequentially formed over the storage element layer 106 (the MTJ).

Referring to FIG. 2B, in a next step, the selector 200, the bottomelectrode 104B, the storage element layer 106, the top electrode 108,and the hard mask layer 208 are selectively etched to form a stackedstructure having inclined sidewalls. For example, the selective etchingprocess includes a reactive ion etching (RIE) process, an ion beametching (IBE) process, or combinations thereof. In some embodiments,after the selective etching process, the inclined sidewalls 200SD of theselector 200 are aligned with the inclined sidewalls 106SD of thestorage element layer 106.

Referring to FIG. 2C, after performing the selective etching process, aliner layer 110 is conformally formed over the first interconnectionstructure 102 and over the hard mask layer 208. Thereafter, apassivation layer 112 is formed on the liner layer 110 and over thefirst interconnection structure 102. In some embodiments, thepassivation layer 112 is formed to surround the selector 200, the bottomelectrode 104B, the storage element layer 106, the top electrode 108,and the hard mask layer 208. Referring to FIG. 2D, in a subsequent step,portions of the passivation layer 112 and portions of the liner layer112 are removed to form an opening (not shown) revealing a surface ofthe hard mask layer 208 (metal hard mask). Thereafter, a secondinterconnection structure 114 is formed in the opening over the hardmask layer 208, and the second interconnection structure 114 iselectrically and physically connected to the hard mask layer 208. Afterforming the second interconnection structure 114, a semiconductor deviceSM2 according to some embodiments of the present disclosure isaccomplished.

As illustrated in FIG. 2D, the semiconductor device SM2 includes astorage element layer 106 and a selector 200 electrically coupled to thestorage element layer 106. For example, the selector 200 is electricallyconnected to the storage element layer 106 through the bottom electrode104B. In some embodiments, the first interconnection structure 102 iselectrically connected to the first conductive layer 202 of the selector200. In certain embodiments, the second interconnection structure 114 iselectrically connected to the storage element layer 106 (the MTJ)through the hard mask layer 208 (metal hard mask) and the top electrode108. In the semiconductor device SM2, by using the selector 200electrically coupled to the storage element layer 106 (the MTJ), adevice that enables full control of leakage current, threshold voltageand maximum current density independently can be achieved.

FIG. 3A to FIG. 3F are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with some otherembodiments of the present disclosure. The method illustrated in FIG. 3Ato FIG. 3F is similar to the method illustrated in FIG. 2A to FIG. 2D.Therefore, the same reference numerals are used to refer to the same orliked parts, and its detailed description will be omitted herein.

Referring to FIG. 3A, a first interconnection structure 102 is provided.The first interconnection structure 102 may include an insulating layer102A and a conductive via 102B disposed therein. After providing thefirst interconnection structure 102, a selector 200 is formed over theconductive via 102B of first interconnection structure 102. For example,in one embodiment, the selector 200 is formed by sequentially depositinga first conductive layer 202, insulating layers 204 (a first insulatinglayer 204A, a second insulating layer 204B and a third insulating layer204C) and a second conductive layer 206 stacked up in sequence.Thereafter, the first conductive layer 202, the insulating layers 204and the second conductive layer 206 are selectively etched to form theselector 200 having inclined sidewalls. In some embodiments, apassivation layer 207 is formed on the first interconnection structure102 to surround the selector 200. In certain embodiments, aplanarization process (e.g. a chemical-mechanical planarization (CMP)process) is performed on the passivation layer 207 so that a top surfaceof the passivation layer 207 is aligned with a top surface of theselector 200. The passivation layer 207 may be formed of the same methodand be made of materials similar to that of the passivation layer 112described in previous embodiments. Therefore, the details of thepassivation layer 207 will be omitted herein.

Referring to FIG. 3B, in a subsequent step, a connecting via 113B and aninsulating layer 113A is formed over the selector 200 and thepassivation layer 207. For example, the connecting via 113B iselectrically and physically connected to the second conductive layer 206of the selector 200, whereby the insulating layer 113A is surroundingthe connecting via 113B. The insulating layer 113A may be formed by anysuitable method, such as chemical vapor deposition (CVD), or the like.The connecting via 113B may be formed by patterning the insulting layer113A to form openings; filling the openings with conductive materials;and performing a planarization step (e.g. chemical-mechanicalplanarization; CMP) to form the connecting via 113B having a top surfacethat is levelled with a top surface of the insulating layer 113A.

Referring to FIG. 3C, after forming the connecting via 113B and theinsulating layer 113A, an insulating layer 104A and a bottom electrode104B embedded in the insulating layer 104A are formed over theconnecting via 113B. For example, the bottom electrode 104B iselectrically and physically connected to the connecting via 113B.Subsequently, a storage element layer 106 (the MTJ) inclusive of areference layer 106A, a tunnel barrier layer 106B and a free layer 106Cis formed over the bottom electrode 104B. Thereafter, a top electrode108 and a hard mask layer 208 are sequentially formed over the storageelement layer 106 (the MTJ).

Referring to FIG. 3D, in some embodiments, the storage element layer106, the top electrode 108, and the hard mask layer 208 are selectivelyetched to form a stacked structure having inclined sidewalls. Forexample, the selective etching process includes a reactive ion etching(RIE) process, an ion beam etching (IBE) process, or combinationsthereof. Referring to FIG. 3E, after performing the selective etchingprocess, a liner layer 110 is conformally formed over the insulatinglayer 104A and over the hard mask layer 208. Thereafter, a passivationlayer 112 is formed on the liner layer 110 and over the insulating layer104A. In some embodiments, the passivation layer 112 is formed tosurround the storage element layer 106 (the MTJ), the top electrode 108and the hard mask layer 208.

Referring to FIG. 3F, in a subsequent step, portions of the passivationlayer 112 and portions of the liner layer 112 are removed to form anopening (not shown) revealing a surface of the hard mask layer 208(metal hard mask). Thereafter, a second interconnection structure 114 isformed in the opening over the hard mask layer 208, and the secondinterconnection structure 114 is electrically and physically connectedto the hard mask layer 208. After forming the second interconnectionstructure 114, a semiconductor device SM3 according to some embodimentsof the present disclosure is accomplished.

As illustrated in FIG. 3F, the semiconductor device SM3 includes astorage element layer 106 and a selector 200 electrically coupled to thestorage element layer 106. For example, the selector 200 is electricallyconnected to the storage element layer 106 through the bottom electrode104B and the connecting vias 113B. In some embodiments, the firstinterconnection structure 102 is electrically connected to the firstconductive layer 202 of the selector 200. In certain embodiments, thesecond interconnection structure 114 is electrically connected to thestorage element layer 106 (the MTJ) through the hard mask layer 208(metal hard mask) and the top electrode 108. In the semiconductor deviceSM3, by using the selector 200 electrically coupled to the storageelement layer 106 (the MTJ), a device that enables full control ofleakage current, threshold voltage and maximum current densityindependently can be achieved.

In the above-mentioned embodiments, the storage element layer 106 andthe selector 200 may be disposed on top or below one another. In otherwords, the storage element layer 106 and the selector 200 are disposedalong the same axis, and is overlapped with one another. Furthermore,the storage element layer 106 and the selector 200 may be connected toone another through electrodes (top or bottom electrodes 108 or 104B),and optionally though connecting vias 113B. However, the disclosure isnot limited thereto. In some alternative embodiments, the storageelement layer 106 and the selector 200 may be disposed on differentaxis, and are non-overlapped with one another. Such embodiment isdescribed with reference to FIG. 4A to FIG. 4E.

FIG. 4A to FIG. 4E are schematic sectional views of various stages in amethod of forming a semiconductor device in accordance with some otherembodiments of the present disclosure. The method illustrated in FIG. 4Ato FIG. 4E is similar to the method illustrated in FIG. 1A to FIG. 1E.Therefore, the same reference numerals are used to refer to the same orliked parts, and its detailed description will be omitted herein.

Referring to FIG. 4A, a first interconnection structure 102 is provided.The first interconnection structure 102 may include an insulating layer102A and a conductive pad 102B disposed therein. The insulating layer102A may be formed by any suitable method, such as chemical vapordeposition (CVD), or the like. The conductive pad 102B may be formed bypatterning the insulting layer 102A to form openings; filling theopenings with conductive materials; and performing a planarization step(e.g. chemical-mechanical planarization; CMP) to form the conductive pad102B having a top surface that is levelled with a top surface of theinsulating layer 102A.

Referring to FIG. 4B, a bottom electrode 104B, a storage element layer106 and a top electrode 108 are sequentially formed over the conductivepad 102B of the first interconnection structure 102. For example, thebottom electrode 104B is disposed on the conductive pad 102B, and iselectrically and physically connected to the conductive pad 102B. Thestorage element layer 106 (the MTJ) inclusive of a reference layer 106A,a tunnel barrier layer 106B and a free layer 106C is formed over thebottom electrode 104B. Furthermore, a top electrode 108 is formed overthe storage element layer 106, whereby the top electrode 108 isconnected to the free layer 106C of the storage element layer 106.Thereafter, the bottom electrode 104B, the storage element layer 106 andthe top electrode 108 are selectively etched to form a stacked structurehaving inclined sidewalls. In a subsequent step, a passivation layer112A is formed over the first interconnection structure 102 to surroundand cover the bottom electrode 104B, the storage element layer 106 andthe top electrode 108. The passivation layer 112A may be formed by thesame method and be made of materials similar to that of the passivationlayer 112 described in previous embodiments. Therefore, the details ofthe passivation layer 112A will be omitted herein.

Referring to FIG. 4C, after forming the passivation layer 112A, thepassivation layer 112A is etched to form an opening (not shown)revealing a top surface of the conductive pad 102B, and etched to formanother opening (not shown) revealing a top surface of the top electrode108. The openings are then filled with conductive materials to formconnecting vias 210A located in the openings. Thereafter, aplanarization step (e.g. chemical-mechanical planarization; CMP) may beperformed so that the connecting vias 210A have top surfaces that areleveled with a top surface of the passivation layer 112A. In theexemplary embodiment, one of the connecting via 210A (first connectingvia) is disposed on the first interconnection structure 102 aside thestorage element layer 106 (the MTJ), while being electrically andphysically connected to the conductive pad 102B. Furthermore, anotherone of the connecting via 210A (second connecting via) is disposed onthe storage element layer 106, and being electrically and physicallyconnected to the top electrode 108. After forming the connecting vias210A, a conductive line 210B may be formed over the connecting via 210A(second connecting via) and over the passivation layer 112A. Forexample, the conductive line 210B is electrically connected to one ofthe connecting via 210A (second connecting via). In the exemplaryembodiment, the conductive vias 210 and the conductive line 210B includemetals or metal alloys such as one or more of Al, AlCu, Cu, Ti, TiN, W,or the like, and may be formed by electroplating, deposition or thelike.

Referring to FIG. 4D, in a next step, a selector 200 is formed over theconnecting via 210A (first connecting via). For example, in oneembodiment, the selector 200 is formed by sequentially depositing afirst conductive layer 202, insulating layers 204 (a first insulatinglayer 204A, a second insulating layer 204B and a third insulating layer204C) and a second conductive layer 206 stacked up in sequence.Thereafter, the first conductive layer 202, the insulating layers 204and the second conductive layer 206 are selectively etched to form theselector 200 having inclined sidewalls. In some embodiments, theconnecting via 210A (first connecting via) is electrically andphysically connecting the selector 200 to the first interconnectionstructure 102. Furthermore, the selector 200 is electrically coupled tothe storage element layer 106 through the connecting via 210A (firstconnecting via), the conductive pad 102B and the bottom electrode 104B.In a subsequent step, a passivation layer 112B is formed over thepassivation layer 112A to surround and cover the selector 200. Thepassivation layer 112B may be formed by the same method and be made ofmaterials similar to that of the passivation layer 112 described inprevious embodiments. Therefore, the details of the passivation layer112B will be omitted herein.

Referring to FIG. 4E, after forming the passivation layer 112B, thepassivation layer 112B is etched to from an opening (not shown)revealing a top surface of the conductive line 210B, and etched to formanother opening (not shown) revealing a top surface of the secondconductive layer 206. The openings are then filled with conductivematerials to form connecting vias 212A located in the openings.Thereafter, a planarization step (e.g. chemical-mechanicalplanarization; CMP) may be performed so that the connecting vias 212Ahave top surfaces that are leveled with a top surface of the passivationlayer 112B. In the exemplary embodiment, one of the connecting via 212A(first connecting via) is disposed on the conductive line 210B aside theselector 200, while being electrically and physically connected to theconductive line 210B. Furthermore, another one of the connecting via212A (second connecting via) is disposed on the selector 200, and beingelectrically and physically connected to the second conductive layer 206of the selector 200. After forming the connecting vias 212A, conductivelines 212B may be formed over the connecting vias 212A and over thepassivation layer 112B. For example, the connecting vias 212A andconductive lines 212B may constitute a second interconnection layer ofthe device. After forming the conductive lines 212B, a semiconductordevice SM4 according to some embodiments of the present disclosure isaccomplished.

As illustrated in FIG. 4E, the semiconductor device SM4 includes astorage element layer 106 and a selector 200 electrically coupled to thestorage element layer 106. For example, the selector 200 is electricallyconnected to the storage element layer 106 through the connecting via210A (first connecting via), the conductive pad 102B and the bottomelectrode 104B. In some embodiments, the first interconnection structure102 is electrically and physically connected to the storage elementlayer 106 (the MTJ) through the bottom electrode 104B. In certainembodiments, the second interconnection structure (212A, 212B) iselectrically and physically connected to the second conductive layer 206of the selector 200. Furthermore, the storage element layer 106 and theselector 200 are disposed on different axis over the firstinterconnection structure 102, and are non-overlapped with one another.In the semiconductor device SM4, by using the selector 200 electricallycoupled to the storage element layer 106 (the MTJ), a device thatenables full control of leakage current, threshold voltage and maximumcurrent density independently can be achieved.

In the above-mentioned embodiments, the semiconductor devices (SM1-SM4)are shown to include a first interconnection structure 102 for providinginterconnection. Although the first interconnection structure 102 isillustrated to include a single layer of conductive via and/orconductive pad embedded within an insulating layer, however, thedisclosure is not limited thereto. The first interconnection structures102 will be described in more detail with reference to FIG. 5.

FIG. 5 is a schematic sectional view of a semiconductor device inaccordance with some embodiments of the present disclosure. Thesemiconductor device SM1′ illustrated in FIG. 5 is similar to thesemiconductive device SM1 illustrated in FIG. 1E. Therefore, the samereference numerals are used to refer to the same or liked parts, and itsdetailed description is omitted herein.

Referring to FIG. 5, the semiconductor device SM1′ may include asubstrate 100, a device region 101, a first interconnection structure102, a bottom electrode 104B, a storage element layer 106, a topelectrode 108, a selector 200, a hard mask layer 208 and a secondinterconnection structure 114. In some embodiments, the substrate 100 isa semiconductor substrate, such as a bulk semiconductor, asemiconductor-on-insulator (SOI) substrate, or the like. The substrate100 may be doped (e.g., with a p-type or an n-type dopant) or undoped.The substrate 100 may be a wafer, such as a silicon wafer. Generally,the SOI substrate is a layer of a semiconductor material formed on aninsulator layer. The insulator layer is, for example, a buried oxide(BOX) layer, a silicon oxide layer, or the like. The insulator layer isprovided on a substrate, typically a silicon or glass substrate. Othersubstrates, such as a multi-layered or gradient substrate may also beused. In some embodiments, the substrate 100 includes an elementsemiconductor such as silicon or germanium, a compound semiconductorsuch as silicon carbide, gallium arsenic, gallium phosphide, indiumphosphide, indium arsenide and indium antimonide, an alloy semiconductorsuch as SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP and GaInAsP orcombinations thereof.

In some embodiments, the device region 101 is disposed on the substrate100 in a front-end-of-line (FEOL) process. The device region 101 mayinclude a wide variety of devices. In some alternative embodiments, thedevices include active components, passive components, or a combinationthereof. In some other embodiments, the devices include integratedcircuits devices. The devices are, for example, transistors, capacitors,resistors, diodes, photodiodes, fuse devices, or other similar devices.In an embodiment, the device region 101 includes a gate structure,source and drain regions, and isolation structures such as shallowtrench isolation (STI) structures (not shown). In the device region 101,various N-type metal-oxide semiconductor (NMOS) and/or P-typemetal-oxide semiconductor (PMOS) devices, such as transistors ormemories and the like, may be formed and interconnected to perform oneor more functions. Other devices, such as capacitors, resistors, diodes,photodiodes, fuses and the like may also be formed over the substrate100. The functions of the devices may include memory, processors,sensors, amplifiers, power distribution, input/output circuitry, or thelike.

As illustrated in FIG. 12, the first interconnection structure 102 isdisposed on the device region 101, and the device region 101 is disposedbetween the substrate 100 and the first interconnect structure 102. Insome embodiments, the first interconnect structure 102 includes aplurality of insulating layer 102A, a plurality of conductive vias 102Band a plurality of conductive pads 102C alternately stacked. Theinsulating layers 102A are referred to as an inter-metal dielectric(IMD) layer which may be made of a dielectric material, such as siliconoxide, silicon nitride, silicon oxynitride, a spin-on dielectricmaterial, or a low-k dielectric material. The conductive vias 102B andthe conductive pads 102C include commonly used conductive materials,such as metals or metal alloys including one or more of Al, AlCu, Cu,Ti, TiN, W, and the like. In some embodiments, the conductive via 102Bconnected to the device region 101 may be referred as “via 1” (V1), andthe conductive via 102B connected to the bottom electrode 104B isreferred as “via n” (Vn). There may be a plurality of conductive vias102B and conductive pads 102C disposed in between the V1 and Vnconductive vias 102B, and this may be adjusted based on designrequirement.

After forming the first interconnection structure 102, the same methoddescribed in FIG. 1B to FIG. 1E may be performed to form the bottomelectrode 104B, the storage element layer 106, the top electrode 108,the selector 200, the hard mask layer 208 and the second interconnectionstructure 114 over the first interconnection structure 102. For example,the storage element layer 106 and the selector 200 are disposed betweenthe first interconnection structure 102 and the second interconnectionstructure 114 in the back-end-of-line (BEOL) process. After forming thesecond interconnection structure 114, a semiconductor device SM1′according to some embodiments of the present disclosure is accomplished.

As illustrated in FIG. 5, the semiconductor device SM1′ includes astorage element layer 106 and a selector 200 electrically coupled to thestorage element layer 106. For example, the selector 200 is electricallyconnected to the storage element layer 106 through the top electrode108. In some embodiments, the first interconnection structure 102 iselectrically connected to the storage element layer 106 (the MTJ)through the bottom electrode 104B. In certain embodiments, the secondinterconnection structure 114 is electrically connected to the selector200 through the hard mask layer 208 (metal hard mask). In thesemiconductor device SM1′, by using the selector 200 electricallycoupled (in series) to the storage element layer 106 (the MTJ), a devicethat enables full control of leakage current, threshold voltage andmaximum current density independently can be achieved.

In the above-mentioned embodiments, the semiconductor device includes astorage element layer and a selector electrically coupled to the storageelement layer. The selector includes first and second conductive layershaving the selected work function, and includes insulating layersrespectively made of low and high band gap materials. As such, bettercontrol of the on/off current of the selector may be achieved, and theselector enables full control of the leakage current as the currentpassing through a selected cell exceeds the residual leakage. By usingsuch a selector that is electrically coupled to the storage elementlayer, a semiconductor device that enables full control of leakagecurrent, threshold voltage and maximum current density independently canbe achieved.

In accordance with some embodiments of the present disclosure, asemiconductor device includes a storage element layer and a selector.The selector is electrically coupled to the storage element layer, andincludes a first insulating layer, a second insulating layer and a thirdinsulating layer, a first conductive layer and a second conductivelayer. The first insulating layer, the second insulating layer and thethird insulating layer are stacked up in sequence, wherein the secondinsulating layer is sandwiched in between the first insulating layer andthe third insulating layer, and the first insulating layer and the thirdinsulating layer include materials with higher band gap as compared witha material of the second insulating layer. The first conductive layer isconnected to the first insulting layer, and the second conductive layeris connected to the third insulating layer.

In accordance with some other embodiments of the present disclosure, asemiconductor device includes a first interconnection structure, amagnetic tunnel junction, a selector and a second interconnectionstructure. The first interconnection structure is disposed on asubstrate. The magnetic tunnel junction and the selector are disposed onthe first interconnection structure, wherein the magnetic tunneljunction is electrically coupled to the selector. The selector includesa first conductive layer, a second conductive layer, two high band gapmaterial layers and a low band gap material layer. The first conductivelayer is electrically connected to the first interconnection structure.The second conductive layer is disposed over the first conductive layerand electrically connected to the first interconnection structure. Thetwo high band gap material layers are disposed in between the firstconductive layer and the second conductive layer. The low band gapmaterial layer is disposed in between the two high band gap materiallayers, wherein a ratio of a thickness of each of the two high band gapmaterial layers relative to a thickness of the low band gap materiallayer is 1:1 to 1:50. The second interconnection structure is disposedover the magnetic tunnel junction and the selector, and electricallyconnected to the magnetic tunnel junction and the selector.

In accordance with yet another embodiment of the present disclosure, amethod of forming a semiconductor device is described. The methodincludes the following steps. A storage element layer and a selector areformed over a first interconnection structure, wherein the storageelement layer is electrically coupled to the selector. A method offorming the selector includes the following steps. A first conductivelayer is formed over the first interconnection structure. A firstinsulating layer, a second insulating layer and a third insulating layerare sequentially formed and stacked over the first conductive layer,wherein the first insulating layer and the third insulating layerinclude materials with higher band gap as compared with a material ofthe second insulating layer. The second conductive layer is formed overthe third insulating layer. A second interconnection structure is formedover the storage element layer and the selector

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a storageelement layer; and a selector electrically coupled to the storageelement layer, wherein the selector comprises: a first insulating layer,a second insulating layer and a third insulating layer stacked up insequence, wherein the second insulating layer is sandwiched in betweenthe first insulating layer and the third insulating layer, and the firstinsulating layer and the third insulating layer include materials withhigher band gap as compared with a material of the second insulatinglayer; a first conductive layer connected to the first insulating layer;and a second conductive layer connected to the third insulating layer.2. The semiconductor device according to claim 1, wherein the firstinsulating layer and the third insulating layer include materialsselected from the group consisting of silicon nitride (SiN), siliconoxide (SiO₂), aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃) and hafniumsilicate (HfSiO_(x)).
 3. The semiconductor device according to claim 1,wherein the second insulating layer includes a material selected fromthe group consisting of titanium oxide (TiO₂), strontium titanate(SrTiO₃), barium titanate (BaTiO₃), zirconium oxide (ZrO₂), hafniumoxide (HfO₂) and hafnium titanate (HfTiO_(x)).
 4. The semiconductordevice according to claim 1, wherein the storage element layer comprisesa reference layer, a free layer and a tunnel barrier layer located inbetween the reference layer and the free layer.
 5. The semiconductordevice according to claim 4, further comprising a top electrodeconnected to the free layer and a bottom electrode connected to thereference layer, wherein the selector is disposed above the storageelement layer and electrically connected to the storage element layerthrough the top electrode.
 6. The semiconductor device according toclaim 4, further comprising a top electrode connected to the free layerand a bottom electrode connected to the reference layer, wherein theselector is disposed below the storage element layer and electricallyconnected to the storage element layer through the bottom electrode. 7.The semiconductor device according to claim 1, wherein the firstconductive layer and the second conductive layer comprises a metalhaving a work function of 3.8 eV to 4.6 eV.
 8. A semiconductor device,comprising: a first interconnection structure disposed on a substrate; amagnetic tunnel junction and a selector disposed on the firstinterconnection structure, wherein the magnetic tunnel junction iselectrically coupled to the selector, and wherein the selectorcomprises: a first conductive layer electrically connected to the firstinterconnection structure; a second conductive layer disposed over thefirst conductive layer and electrically connected to the firstinterconnection structure; two high band gap material layers disposed inbetween the first conductive layer and the second conductive layer; anda low band gap material layer disposed in between the two high band gapmaterial layers, wherein a ratio of a thickness of each of the two highband gap material layers relative to a thickness of the low band gapmaterial layer is 1:1 to 1:50; a second interconnection structuredisposed over the magnetic tunnel junction and the selector, andelectrically connected to the magnetic tunnel junction and the selector.9. The semiconductor device according to claim 8, wherein a ratio of thethickness of each of the two high band gap material layers relative tothe thickness of the low band gap material layer is 1:2 to 1:40.
 10. Thesemiconductor device according to claim 8, further comprising a topelectrode disposed on the magnetic tunnel junction and a bottomelectrode disposed below the magnetic tunnel junction, wherein themagnetic tunnel junction is electrically coupled to the selector throughthe top electrode.
 11. The semiconductor device according to claim 10,further comprising a liner layer covering sidewalls of the magnetictunnel junction and sidewalls of the selector, and a passivation layersurrounding the liner layer.
 12. The semiconductor device according toclaim 8, further comprising a top electrode disposed on the magnetictunnel junction and a bottom electrode disposed below the magnetictunnel junction, wherein the magnetic tunnel junction is electricallycoupled to the selector through the bottom electrode.
 13. Thesemiconductor device according to claim 8, further comprising aconnecting via physically connecting the selector to the firstinterconnection structure, and wherein the magnetic tunnel junction isdisposed on the first interconnection structure aside the connectingvia.
 14. The semiconductor device according to claim 8, wherein the twohigh band gap material layers include materials selected from the groupconsisting of silicon nitride (SiN), silicon oxide (SiO₂), aluminumoxide (Al₂O₃), yttrium oxide (Y₂O₃) and hafnium silicate (HfSiO_(x)).15. The semiconductor device according to claim 8, wherein the low bandgap material layer includes a material selected from the groupconsisting of titanium oxide (TiO₂), strontium titanate (SrTiO₃), bariumtitanate (BaTiO₃), zirconium oxide (ZrO₂), hafnium oxide (HfO₂) andhafnium titanate (HfTiO_(x)).
 16. A method of forming a semiconductordevice, comprising: forming a storage element layer and a selector overa first interconnection structure, wherein the storage element layer iselectrically coupled to the selector, and forming the selectorcomprises: forming a first conductive layer over the firstinterconnection structure; sequentially forming a first insulatinglayer, a second insulating layer and a third insulating layer stackedover the first conductive layer, wherein the first insulating layer andthe third insulating layer include materials with higher band gap ascompared with a material of the second insulating layer; and forming asecond conductive layer over the third insulating layer; and forming asecond interconnection structure over the storage element layer and theselector.
 17. The method according to claim 16, further comprising:forming a bottom electrode over the first interconnection structure;forming the storage element layer on the bottom electrode; forming a topelectrode over the storage element layer; and forming the selector overthe top electrode so that the first conductive layer is connected to thetop electrode.
 18. The method according to claim 16, further comprising:forming the selector over the first interconnection structure; forming abottom electrode over the selector so that the bottom electrode iselectrically connected to the second conductive layer of the selector;forming the storage element layer on the bottom electrode; and forming atop electrode over the storage element layer.
 19. The method accordingto claim 16, wherein forming the storage element layer comprises:forming a reference layer over the first interconnection structure;forming a tunnel barrier layer over the reference layer; and forming afree layer over the tunnel barrier layer.
 20. The method according toclaim 16, further comprising: forming a liner layer covering sidewallsof the storage element layer and sidewalls of the selector; and forminga passivation layer surrounding the liner layer.